The dramatic increase in the prevalence of overweight among children and adolescents has led to obesity prevention becoming a major national health priority. Although schools offer a logical choice for a setting to reach a great number of youth, these disappointing findings may be due in part to the difficulties of implementing interventions in school settings where competing demands for time have made it difficult to add anything other than academics to the school day. After-school programs may provide promise for preventing child obesity because there are fewer bureaucratic obstacles and curricular inflexibilities. A three-year group-randomized controlled trial will be conducted with random assignment at the school level . The study will use a nested cross section design with a baseline year, and two subsequent intervention years.